Internal gear pumps have long been known as pumps which discharge sucked liquid against pressure, and particularly have been popular as hydraulic source pumps or oil feed pumps.
An internal gear pump includes, as main active components, a spur gear type inner rotor with teeth on its outer surface, and an annular outer rotor with teeth on its inner surface which has almost the same width as the inner rotor. A casing, which has flat inner surfaces facing both side faces of these rotors with a small gap, is provided to house the rotors. The number of teeth of the inner rotor is usually one smaller than that of the outer rotor, and the rotors rotate with their teeth meshed with each other, like power transmission gears. As the groove area changes with this rotation, the liquid trapped in the grooves is sucked or discharged so that the function as a pump is performed. When one of the inner and outer rotors is driven, the other rotor, meshed with it, rotates as well. Since the center of rotation is different between the rotors, each rotor must be pivotally supported in a rotatable manner individually. The casing has at least one so-called suction port and at least one so-called discharge port as openings to flow channels communicated with the outside. The suction port is designed to communicate with a groove whose volume increases and the discharge port is designed to communicate with a groove whose volume decreases. As for rotor profiles, typically, the outer rotor profile includes an arc and the inner rotor teeth are trochoidal teeth.
Since the internal gear pump rotates with its inner rotor and outer rotor meshed, when one rotor is driven, the other rotor rotates as well. When a motor part is integral with the outer surface of a-pump part and the rotator of the motor part is integral with the outer rotor and the motor part drives the outer rotor, this structure can be shorter than a structure in which the pump part and the motor part are arranged in series along the axial direction and is thus suitable for a compact pump.
An example of this type of internal gear pumps is the one disclosed in Japanese Patent Application Laid-Open Publication No. H2-277983 (Patent Document 1). According to Patent Document 1, the internal gear pump includes an internal gear which combines an outer gear (equivalent to an outer rotor) having a rotor on its outer surface to face and contact a stator fitted in a motor casing, with a given gap inside the stator in the radial direction, and an inner gear (equivalent to an inner rotor) to mesh with this outer gear, wherein both end faces of the internal gear are liquid-tightly closed by end plates and one of the end plates has a suction port and a discharge port which communicate with the internal gear. The end plates include a front casing and a rear casing; disc thrust bearings are disposed between the casings and both sides of the internal gear pump; and both sides of the outer gear are supported by the thrust bearings; both ends of a support shaft are fixed to the casings and the inner gear is rotatably supported by the support shaft through a radial bearing; and also a liquid feed channel is provided to allow some of the pressurized liquid on the discharge side to flow between the rotor and stator and lubricate the bearings and flow back to the suction side.
Patent Publication 1: Japanese Patent Application Laid-Open Publication No. H2-277983